1. Field of the Invention
The present invention relates to an electron emission device for use as a source for electrons in an electron microscope, an electron beam exposure apparatus, a planar image display, or any of various other applications using an electron beam, and a method of manufacturing such an electron emission device.
2. Prior Art
Hot cathodes for emitting electrons by thermionic emission are used as the source for electrons in various electron beam devices such as an electron microscope, an electron beam exposure apparatus, or a planar image display. The hot cathode requires a heater for heating the cathode itself, and hence causes a loss of energy because of the heating of the cathode.
Recent years have seen the advent of an electron emitter, known as a cold cathode, which does not depend on heat for electron emission. There have been proposed various electron emission devices incorporating the cold cathode. According to one electron emission device, a PN junction is reverse-biased to bring about an electron avalanche breakdown for electron emission Another electron emission device is of the MIM type which has a three-layer structure composed of a metal layer, an insulation layer, and a metal layer. When a voltage is applied between the metal layers, electrons are forced to pass through the insulation layer due to the tunnel effect, and emitted out of a metal layer surface. Still another electron emission device, which operates on the principle of field emission, has a specially shaped metal surface to which a voltage is applied to develop a localized highly intensive electric field which emits electrons out of the metal surface.
One field-emission-type electron emission device has a cathode emitter whose end is machined into a sharply pointed needle tip having a curvature of several hundreds nm or smaller so that a concentrated electric field of about 10.sup.7 V/cm will be developed at the pointed needle tip. The field-emission-type electron emission device of this type offers the following advantages:
(1) It has a high current density.
(2) Any consumption of electric energy is very small as the cathode emitter requires no heating.
(3) The device can be used as point and linear sources for electron beams.
A field emission-type electron emission device is shown in Journal of Applied Physics, Vol. 39, No. 7, Page 3504, 1956, for example. FIG. 1(a) of the accompanying drawings shows such a known field-emission-type electron emission device in the process of being manufactured. FIG. 1(b) illustrates the field-emission-type electron emission device as it is completed.
The field-emission-type electron emission device is manufactured as follows: As shown in FIG. 1(a), an electrically conductive film 102, an electrically insulative layer 103, and an electrically conductive film 104 are successively evaporated on an electrically insulative substrate 101. The conductive film 104 and the insulative layer 103 are selectively etched away to produce an array of cavities 105 therein according to a photolithographic process. Thereafter, while the open ends of the cavities 105 are being progressively closed by a suitable material 106 according to the rotary slant evaporation process, a cathode material 107 is evaporated on the conductive film 102 through the open ends of the cavities 105, thereby forming upwardly pointed cathode emitter projections 108 on the conductive film 102 within the cavities 105. Thereafter, the evaporated material 106 is removed, completing the electron emission device as shown in FIG. 1(b).
A power supply 109 is connected to the conductive films 104, 102 such that the conductive film 104 is kept at a positive potential and the conductive film 102 is kept at a negative potential. When a voltage higher than a predetermined voltage that is determined by the cathode material 107 is applied between the conductive films 104, 102, a concentrated electric field is developed which causes the cathode emitter projections 108 to emit electrons.
An effort has been directed to a planar display which comprises an array of such electron emission devices (see Japan Display, 1986, page 512).
Japanese Patent Publication No. 54(1979)-17551 discloses another conventional electron emission device. FIGS. 2(a) through 2(f) of the accompanying drawings show a process of successive steps for manufacturing such a conventional electron emission device.
First, as shown in FIG. 2(a), a thin film 122 of a cathode material is evaporated on one surface of each of a plurality of rectangular, electrically insulative substrates 121, thus producing a plurality of substrates 123. Then, the substrates 123 are superposed to provide a unitary substrate 124, after which the surfaces of the substrate 124 are machine ground. Then, as shown in FIG. 2(b), a metal film 125 is evaporated on one of the wider surfaces of the substrate 124. Electron emission windows 126, which are as narrow as the thin films 122, are defined in the metal film 125 directly over the respective thin films 122 by a photoetching process, as shown in FIG. 2(c). Then, the substrates 123 are separated from each other, and the thin film 122 of each substrate 123 is etched into a cathode emitter 127 having a pointed triangular pattern, as shown in FIG. 2(d). Thereafter, as shown in FIG. 2(e), each substrate 121 is partially chemically eroded away to produce a cavity 128 such that the pointed ends of the cathode emitter 127 are spaced from the substrate 121 and the edge of the metal film 125 along the electron emission window 126 overhangs. As shown in FIG. 2(f), the substrates 123 are superposed again and fixed together, thus producing a thin-film cold-cathode array.
The production of the electron emission device shown in FIGS. 1(a) and 1(b) is disadvantageous in that it is very difficult to control the two simultaneous evaporation processes, i.e., for depositing the material 106 and the cathode emitter projections 108 simultaneously.
With the electron emission device shown in FIGS. 2(a) through 2(f), the thicknesses of the insulative substrates 121 and the thin films 122 must be highly accurate in order to position the electron emission windows 126 and the cathode emitters 127 in accurate alignment with each other. Furthermore, difficulty has been experienced in fixing the substrates 123 with the same degree of accuracy when they are first assembled into the substrate 124 and subsequently put together into the final product.